A hybrid vehicle drive system as disclosed by U.S. Pat. No. 4,533,011 rotates drive wheels under an output torque of either or both an engine and a motor/generator according to the running conditions.
When a continuously variable transmission is used with such a drive system, when an accelerator pedal is released while the vehicle is running at high speed, a drive ratio of the transmission varies when the vehicle begins coasting.
Herein, the drive ratio is the ratio of the rotation speed of an input shaft and output shaft of the transmission.
When the vehicle starts coasting, there is an upshift to a minimum drive ratio, then the continuously variable transmission shifts down while the engine rotation speed remains constant due to the decrease of vehicle speed, and after the drive ratio reaches the maximum drive ratio, this maximum drive ratio is maintained until the vehicle stops.
However, during an upshift, the engine brake power decreases, and during a downshift, the engine brake force increases. Therefore, it is impossible to achieve a stable engine braking force during this alternating pattern of upshift and downshift.